1. Field of the Invention
The present invention relates to the field of high power ultra-high frequency signal generators. It relates more particularly to an amplifying klystron able to deliver a very high ultra-high frequency power and adapted to feed a variable impedance load.
2. Description of the Prior Art
For a better understanding of the problem and the solutions of the prior art, there will first of all be described, with reference to FIG. 1, the schematical structure and the operation of an amplifying klystron and, with reference to FIG. 2, the performances which may be reached with such a device and the existing limitations in the case where it is called on to operate across a variable load.
FIG. 1 shows very schematically the conventional structure of an amplifying klystron. This klystron tube comprises, inside a vacuum enclosure not shown, an electron gun of which only the cathode 1 and the accelerating anode 2 have been shown schematically, which generates and projects an electron beam 3 towards a collector 4 placed at the other end of the tube. A succession of cavity resonators is placed along the beam so as to interact with this beam through their respective electric fields. An input cavity 5, two intermediate cavities 6 and 7 and an output cavity 8 can be seen. The assembly is surrounded as far as possible by a magnetic winding 9 providing focusing of the beam. The number of intermediate cavities is variable, they may possibly be tuneable and their characteristics as well as those of the input and output cavities are determined by a man skilled in the art depending on the performances to be obtained. For construction thereof, reference may be made to conventional works on amplifying klystrons.
A coupling circuit having a terminal 10 feeds to the input cavity the signal to be amplified coming from an ultra-high frequency oscillator 11. The electric field of this input cavity 5 influences the electrons of the beam in the first interaction gap by modulating the speed of the electrons of this beam. Resonators 6 and 7 are excited by the modulated beam and react on the electron beam 3 while producing favorable grouping of the electrons in the interaction gap of the output resonator 8. The grouped electrons passing through the interaction gap of the output cavity 8 excite this cavity and the ultra high frequency power is extracted from this resonator by means of an output coupling circuit 12 which comprises, in the embodiment shown, an endpiece 13 forming a wave guide whose first end gives on to an opening 14 in the wall of cavity 8 and whose other end is sealed to the vacuum by a window 15 which is transparent to the ultra high frequency electro-magnetic radiation generated by the tube.
The above described assembly (except for oscillator 11) forms a power amplifying klystron, which is delivered by the manufacturer to the users while specifying well defined conditions of use and cooling. It is in fact clear, if we consider for example a power amplifying klystron intended to supply continuously an electro-magnetic radiation of a power of 500 kilowatts at 500 MHz, that the cooling conditions must be particularly well complied with. It will more especially be advisable to cool the collector 4 which receives the electron beam. Cooling circuits are generally provided as well for the different cavities and in any case for the output cavity at the level of which there are greater chances that the electrons diverge and impact on the projections 16 of the interaction gap. So at least this output cavity is generally provided with a jacket 17 in which is provided a flow of cooling fluid, for example deionized water.
It is then up to the user to connect to this apparatus the device to which he desires to apply the ultra high frequency energy. For this, the user connects to end piece 13 wave guide 20 directing the energy of the klystron towards the load, for example a plasma which it is desired to heat. The user must also adapt the load so that no energy or only a very small amount of energy (less than 1% of the emitted energy) is fed back towards the klystron, that is to say that the ratio of standing waves in guide 20 must be substantially equal to 1. If the load to be energized is a fixed stable load, this condition may be fulfilled by suitably selecting guide 20, its coupling coefficient, its dimensions, etc. On the other hand, if the load has an impedance varying in time, which is for example the case in micro wave heating or plasma heating, the standing wave ratio may vary considerably. It may rise to values as high as 3/1 or even 4/1, with phase changes difficult to foresee. Consequently, in the prior art, when a power klystron is coupled across a load and when it is desired to cause this klystron to operate in its high efficiency zone, a circulator is disposed between the output wave guide 20 and the load.
A circulator is, as is well known, a device for deflecting the ultra high frequency energy reflected by a load so that it does not come back into its incident path. It is generally possible to manufacture a circulator giving satisfactory results and avoiding the return of energy to the emitting klystron. The disadvantage of such circulators resides essentially in their cost which becomes extremely high when the powers involved are high. Efforts are therefore made to do away with these circulators. As will be made clear hereafter, that can only be done conventionally at the price of using the klystron in a low efficiency zone.
FIG. 2 shows a graph characterizing the efficiency of a klystron. The value VC/VO of the normalized voltage at the level of the interaction gap of the output cavity is shown in abscissa, VC being the voltage at the terminals of this interaction gap and VO the acceleration voltage of the electron beam 3. The ordinates show the value IC/IO of the normalized current in the output cavity, IC being the current in this output cavity and IO the current of the electron beam. In such a graph, constant values of the IC.VC/IO.VO ratio characterize given efficiencies of the tube and correspond to hyperbolic shaped curves. In the Figure have been shown curves 30 to 35 characteristic of efficiencies going from 30% to 80%. Depending on the coupling of the output cavity, that is to say more especially on the dimensions of opening 14, for a given tube, the value of VC may be more or less large. Curves 40 and 41 show how IC varies as a function of VC, that is to say depending on the coupling of the output cavity for tubes having given characteristics. Curve 40 corresponds to a klystron with a low perveance and curve 41 to a klystron with higher perveance.
It can be seen that to increase the efficiency, it is desirable to increase the value of the voltage VC as much as possible. There exists however a limit for the maximum values of the peak voltages VC. This limit is reached when the peak voltage VC is substantially equal to the acceleration voltage VO of the beam. In fact, if VC becomes too much greater than VO, a part at least of the grouped electrons risks being braked excessively and sent back in the general direction of the cathode. These returning electrons will be generally defocused and will strike the internal walls of the klystron, particularly at the level of the projections of the output cavity which risk being heated considerably and even melting, which causes the destruction of the klystron tube. In the case of curves 40 and 41, there have been shown the respectively limits VC1 and VC2 of the maximum values of the voltage VC at the terminals of the interaction gap for avoiding this phenomenom.
If the load is perfectly matched to the klystron and if the energy reflected by this load is practically zero, the klystron may effectively operate in these zones of maximum efficiency, i.e. for the voltages VC1 and VC2 shown in FIG. 2. If now the klystron is used without a circulator across a variable load and if the energy reflected may for example be of the order of 10% of the emitted energy (standing wave ratio equal to 2), this results in the appearance of a voltage V'C which is superimposed on the voltage VC at the terminals of the interaction space, V'C being of the order of 30% of VC. The resulting voltage then risks exceeding the tolerable limit and leading to destruction of the tube. If the variations of the load impedance have a random character, the tube can only be protected by permanent operation under low excitation, leading to low efficiency in all circumstances.